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NW Fishletter #288, May 12, 2011

[4] BuRec Report Catalogs 'Possible' Climate Impacts

Using a combination of original and peer-reviewed research, the Bureau of Reclamation has released a congressionally mandated assessment of future water supplies in eight major river basins in the West, including the Columbia--where precipitation and runoff are actually predicted to rise, along with temperatures, although flow timing is expected to change.

"Water is the lifeblood of our communities, rural and urban economies, and our environment, and small changes in water supplies or the timing of precipitation can have a big impact on all of us," said Interior Secretary Ken Salazar in a news release accompanying the report. He said the document would provide the foundation for understanding long-term impacts of climate change on Western water supplies, and help develop mitigation strategies to maintain a sustainable water policy.

By the 2070s, said the BuRec report, the western U.S. may face temperature increases of 5-7 degrees. Higher levels of precipitation are anticipated in northwestern and north-central basins, but less precipitation is expected in southwestern and south-central regions.

Almost all regions are expected to see a decline in April 1 snowpack, but more snow will likely be found at higher elevations in more northerly parts of the Columbia River Basin--a phenomenon that could mitigate some effects of increasing temperatures.

Model simulations estimate snowpack levels in the Columbia Basin at The Dalles will be 26 percent lower than 1990s levels by the 2020s, when the basin's average air temperature is expected to climb 1.4 degrees. However, overall precipitation is pegged to rise 3 percent.

By the 2050s, a 3.5-degree increase is expected to produce a 39-percent decline in snowpack, but a 6-percent boost in precipitation.

And a 4.7-degree increase in temperature by the 2070s will lead to a 47-percent decrease in snowpack, along with an 8-percent increase in precipitation, the report said.

Mean annual runoff in the Columbia Basin is estimated to go up by 2 percent in the 2020s, 3.7 percent in the 2050s, and 7.5 percent by the 2070s.

However, major changes in flow timing are projected to occur. Simulated December-March runoff at The Dalles would increase by about 10 percent in the 2020s, 19 percent by the 2050s, and 27 percent by the 2070s.

Since the added precipitation in northern basins is expected to increase high-elevation snowpack, the simulations estimate April-July runoff actually rising by 2 percent in the 2020s and 4 percent by the 2050s, but back down to 2.4 percent in the 2070s.

Hotter days to come are likely to affect fish populations and other aquatic species, creating more thermal stress and barriers to passage from warming water that adult salmon and steelhead will have to navigate on their way to spawning beds.

Another report cited by the BuRec document said these periods of thermal stress could multiply four times by the 2080s.

Warming waters could also help invasive species like quagga mussels to gain a foothold in local waters. They already plague water projects in the Colorado Basin, including Hoover Dam.

The report cited several other studies suggesting hydro operations could still cope with an overall increase of 3 degrees. One, by Lee et al. in 2009, stated, "Under existing operating criteria, satisfaction of flood control objectives would be prioritized and lead to decreased storage trends and hydropower production."

However, the Lee study said if operating adjustments were made, "it would seem possible to rebalance flood control and other system operating objectives so that many of the impacts to water supply and hydropower generation could be reduced while providing comparable levels of flood control to those produced by current flood control practices."

But earlier runoff might reduce the amount of water currently available for irrigation. Increased spring precipitation has the potential to boost plant growth and increase forest fire risk.

The bureau said it will refine these potential impacts in more detailed studies to come, focusing on the Yakima and Snake rivers.

But in some respects, not much has changed on the climate-modeling front for some time, nor have the scary numbers.

In April 2004, then-University of Washington researcher Phil Mote told the Northwest Power and Conservation Council about a hydrologic model developed at the UW, using a moderate warming rate, which estimated 35 percent less snowpack in warmer areas like the Cascades and southern Idaho by the 2050s. By the 2090s, the model predicted 47 percent less snowpack, with very little snow left in the Oregon Cascades--the same estimate of snowpack decline cited in the new BuRec report.

Last year, a modeling effort by UW's Climate Impact Group was described in the Journal of Water Resources Planning and Management. It took a broad-brush approach, using a monthly-time series, and found dam operators could begin refilling some reservoirs up to a month earlier than current operations call for, and hold water longer to cope with the warmer climate they expect to see 50 years from now.

With the expectation of less snow and peak spring flows, overall flood risk would decline in some places, said Se-Yeun Lee, a UW post-doctoral student who worked on the analysis for her doctorate. "With reduced flood risk we can release less water and refill earlier," she said. "As a result, we can supply more hydropower in summer and more storage for other needs like fish flows."

However, their analysis could be overestimating future snowpack loss and timing of peak spring flows, according to other UW atmospheric researchers (Stoelinga et al.), who pegged snowpack losses in the Cascades in the range of only 2 percent per decade. They said it's likely snowpack declines are mitigated by a cool northern Pacific Ocean, which may dampen overall effects of global warming in the Northwest. -B. R.

The following links were mentioned in this story:

Congressionally mandated assessment

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